This invention relates to time base correctors for use with video tape recorders which record non-phased 625 line PAL colour television signals or monochrome 625 line television signals.
The process of time base correction is necessary with video tape recorders because of the timing perturbations created by the mechanical components of the video tape recorder, i.e. the tape moving past a scanning head driven by some type of electric motor. These output timing perturbations which appear on the output video signal must be removed if the signal is to meet full broadcast timing stability specifications or if it is required to superimpose two video signals in some manner in any type of television installation. Different types of analogue time base correctors have been used with the more expensive video tape recorders, but their timing correction range has been limited. The use of a digital storage system enables a much wider timing correction range to be obtained. The storage system would perhaps contain four or more lines of storage and would produce a timing correction window of two or more television lines.
The time base corrector receives the signal from the video tape recorder containing the time base perturbations and produces a signal at the output which is continuous and accurately timed with respect to local synchronizing information.
In many PAL video tape recorders, the complete signal is not recorded directly on the tape using the FM system. Instead, the chrominance information is separated from the luminance by means of a high pass filter, then the low pass filtered luminance information is used to frequency modulate a carrier which is applied to the tape in the normal way, while the chrominance information at 4.43 MHz is heterodyned by a first mixing operation to a lower carrier frequency, around 1 MHz, and is then directly recorded on the tape without using the FM carrier system. Often the frequency used to mix with the incoming chrominance signal to produce the low frequency components to record directly on the tape is not related to either the incoming sub carrier frequency or the horizontal scanning frequency.
The composite signal is recovered again on playback by a second heterodyning operation to bring the chrominance information back up to approximately its original frequency, when it is then added back to the luminance signal. No exact relationship now exists between the new output colour sub carrier frequency of the VTR and the horizontal and vertical scanning frequency of the luminance signal. This is said to be the non-phased colour mode of operation. A signal of this kind does not meet broadcasting standards, neither can two signals of this type be mixed and faded properly as they would in general have different colour sub carrier frequencies.